For vehicles, various electronic key systems are proposed. In one system, in response to a request (transmission request signal) transmitted from an on-vehicle device, a portable device as an electronic key carried by a user of the vehicle transmits a reply signal containing an identification code. If the code that the on-vehicle device receives from the portable device coincides with a code stored in advance in a memory of the on-vehicle device, predetermined operations such as door unlocking/locking and engine startup are performed. Such an electronic key system is generally referred to as a smart key system and a smart keyless entry system (e.g., JP-2000-170420A and U.S. 2003/0095039A1 (JP-2003-157483A)).
In the above electronic key system, desired functions (door unlocking, locking, etc.) are achieved if the user only carries the portable device without performing intentional operations on the portable device. The user therefore often leaves the portable device in a bag, a purse, a coat pocket, or the like.
Therefore, when the user forgets to carry the portable device, he or she becomes aware of not carrying it for the first time when functional operations such as door unlocking are not performed though he or she approaches the vehicle.